In a publication entitled "Generation of light by a scattering medium with negative resonance absorption", Sov. Phys. JETP, Vol. 26, No. 4, April 1968 (pps. 835-839), V. S. Letokhov presents a theoretical analysis of the generation of light by a scattering medium with negative resonance absorption or gain. This analysis requires that a photon mean free path (.LAMBDA..sub.s) be much smaller than all of the dimensions (R) of the active scattering region (equation 1). In a discussion of a condition for a generation threshold, an example is provided for an optically excited spherical distribution of ruby particles (.lambda.=7.times.10.sup.-5 cm) with radius 2.times.10.sup.-4 cm, and the resulting critical radius of the region is shown to be approximately 4 mm. Letokhov also provides a theoretical analysis of scattering particles that are distributed in a gaseous medium with negative absorption, such as a He--Ne or He--Xe gas mixture excited by an electric discharge. The scattering particles are said to effect a non-resonant feedback, while the gaseous active medium effects resonant amplification. The critical effective radius for such a gaseous medium is said to be approximately 1.8 cm. A continuous narrowing of the emission spectrum predicted.
Reference in this regard is also made to an earlier theoretical paper by Letokhov, "Stimulated emission of an ensemble of scattering particles with negative absorption", ZhETF Plasma 5, No. 8, 15, Apr. 1967, (pps. 262-265), wherein the dimensions of the medium are given as R&gt;&gt;.LAMBDA..sub.s &gt;&gt;.lambda. where, as before, R is the dimensions of the medium, .LAMBDA..sub.s is the mean free path of a photon due to scattering, and .lambda. is the wavelength of the photon.
Reference is also made to a publication by Ambartsumyan R. V., Basov N. G., Kryukov P. G. & Letokhov V. S. in Progress in Quantum Electronics (ed. Sanders J. H. & Stevens K. W. H.) 109-185 (Pergamon Press, Oxford, 1970), where a theoretical presentation is made at pages 152-153 of a case when the free path of a photon due to scattering, .LAMBDA..sub.s -1/Q.sub.s N.sub.0, the average dimension of the region occupied by a cloud, R, and the wavelength of the emission .lambda. satisfy the relation EQU R&gt;.LAMBDA..sub.s &gt;.lambda.,
and where the mean distance between the scattering particles is much greater than the wavelength.
One problem that is apparent in the approach of Letokhov is that all of the dimensions of the medium must be much greater than the scattering length. By example, each dimension of the medium may be required to be on the order of a centimeter. These dimensional requirements would preclude the use of the medium for many valuable high spatial resolution applications.
By example, one particularly valuable application which could not be achieved in accordance with the teachings of Letokhov is the formation of a thin layer, coating, or body that included the gain medium. Another example is a sphere or cylinder whose radius was comparable to or smaller than the scattering length.
A further problem is the requirement of providing scattering particles in a gaseous medium, particularly one that is excited by an electrical discharge. This may be difficult to achieve in practice, and may be impractical for most applications.
Reference is also made to an article entitled "Generation of stimulated noncoherent radiation in light-scattering media exhibiting chemical reactions", Sov. J. Quantum Electron. 12(5), May 1982, (pps. 588-594), wherein I. A. Izmailov et al. propose that a feedback resulting from scattering be used to achieve lasing in a disperse reactive medium. The feasibility of chemically pumping the laser is estimated on the basis of calculations of the heterophase burning of a drop of fuel in an oxidizing atmosphere. The reactions between NO and O.sub.3, Ba and S.sub.2 Cl.sub.2, and Ba and N.sub.2 O are specifically calculated.
A laser device based on this approach, if at all possible to realize in a practical sense, would appear to be limited to a narrow range of specialized applications.
Reference is also made to the following three U.S. Patents, all of which disclose and claim inventions that were made by the inventor of the invention disclosed in this patent application: U.S. Pat. No. 5,157,674, issued Oct. 20, 1992, entitled "Second Harmonic Generation and Self Frequency Doubling Laser Materials Comprised of Bulk Germanosilicate and Aluminosilicate Glasses"; U.S. Pat. No. 5,233,621, issued Aug. 3, 1993, which is a division of the previous patent; and U.S. Pat. No. 5,253,258, issued Oct. 12, 1993, entitled "Optically Encoded Phase Matched Second Harmonic Generation Device and Self Frequency Doubling Laser Material Using Semiconductor Microcrystallite Doped Glasses".